Fuel cells are among the most efficient of power generation devices. One type of solid oxide fuel cell (SOFC) generator has a projected 70 percent net efficiency when used in an integrated SOFC-combustion turbine power system in which the turbine combustor is replaced by a SOFC.
Several different fuel cell designs are known. For example, one type of solid oxide fuel cell consists of an inner porous doped-lanthanum manganite tube having an open end and a closed end, which serves as the support structure for the individual cell, and is also the cathode or air electrode of the cell. A thin gas-tight yttria-stabilized zirconia electrolyte covers the air electrode except for a relatively thin strip of an interconnection surface, which is a dense gas-tight layer of doped-lanthanum chromite. This strip serves as the electric contacting area to an adjacent cell or, alternatively, to a power contact. A porous nickel-zirconia cermet layer, which is the anode or fuel electrode, covers the electrolyte, but not the interconnection strip.
Exemplary fuel cells are disclosed in U.S. Pat. No. 4,431,715 to Isenberg, U.S. Pat. No. 4,490,444 to Isenberg, U.S. Pat. No. 4,562,124 to Ruka, U.S. Pat. No. 4,631,138 to Ruka, U.S. Pat. No. 4,748,091 to Isenberg, U.S. Pat. No. 4,791,035 to Reichner, U.S. Pat. No. 4,833,045 to Pollack, et al., U.S. Pat. No. 4,874,678 to Reichner, U.S. Pat. No. 4,876,163 to Reichner, U.S. Pat. No. 5,108,850 to Carlson et al., U.S. Pat. No. 5,258,240 to Di Croce et al., and U.S. Pat. No. 5,273,838 to Draper et al., each of which is incorporated herein by reference.
The air electrode tubes used for solid oxide fuel cells are required to be very straight, for example, with a maximum allowable bow of 2.0 mm over a length of 1.81 m. The process used to make a finished tube consists of several steps. First, organic binders, inorganic powder and water are mixed under high shear to form a paste with suitable rheological properties. This mix is then extruded through a die under high pressure to form the tubular shape of desired cross-sectional geometry. As the tube dries, it becomes rigid such that it can be handled. Conventional air electrode tubes undergo two heating steps. The extruded tubes are first heated horizontally to burn off the organic binders and to develop handling strength. The tubes are then fired to their desired density while hanging vertically.
The primary obstacle in fabricating straight tubes is forming straight green tubes composed of organic binders and the air electrode material. The straightness of the tube prior to sintering dictates in large part the resultant straightness of the sintered tube. In the past, tubes have been extruded onto V-shaped racks and dried in a controlled temperature/humidity chamber. This has been done in an effort to moderate and control the drying rate of the tube such that it would not become bowed. However, this process has been only marginally successful, and the tubes are often severely bowed. If a dried tube is bowed, vertical sintering is conventionally required in order to straighten the air electrode tube in order to correct the problem. At typical air electrode sintering temperatures of 1,500-1,600.degree. C., high temperature creep and gravity work together to straighten the once bowed tube to within allowable limits. However, vertical sintering adds another processing step and does not consistently result in the formation of tubes within the desired straightness tolerance. Accordingly, it would be advantageous to form dried air electrode tubes of sufficient straightness such that subsequent straightening processes are not required.
The present invention has been developed in view of the foregoing, and to address other deficiencies of the prior art.